1. Technical Field
The present invention relates to a semiconductor laser and a method of manufacturing a semiconductor laser.
2. Related Art
As a technique for preventing the deterioration of an end surface due to optical damage in semiconductor lasers, a technique has been proposed in which a window structure that does not absorb laser oscillation light is provided at an end surface.
For example, Japanese Unexamined Patent Publication No. 2006-319120 discloses an improved semiconductor laser that prevents the diffusion of impurities.
FIGS. 16A to 16C show a process of manufacturing the semiconductor laser. First, a ZnO layer 201 is selectively etched and a p-GaAs contact layer 109 where a gain region 004 is to be formed is selectively etched (FIG. 16A). Then, a dielectric film 202 is formed on the entire surface of a wafer (FIG. 16B). Then, a heat treatment (annealing) is performed to diffuse Zn in the ZnO layer 201 into an active layer 104 in a solid phase (FIG. 16C).
In this way, in the semiconductor laser, Zn is diffused from only a portion coming into contact with the p-GaAs contact layer 109. Therefore, the diffusion of impurities is prevented as compared to a structure in which the impurities are diffused with a contact layer provided on the entire surface.
In addition, Japanese Unexamined Patent Publication No. 2006-319120 also discloses a structure in which partition regions including impurities are provided on both sides of a Zn-diffused region, thereby preventing the diffusion of impurities into an active layer in gain regions provided on both sides of a window region due to annealing.
In the semiconductor laser disclosed in Japanese Unexamined Patent Publication No. 2006-319120, as shown in FIGS. 16A, 16B, and 16C, a p-GaAs contact layer 109 is formed only in a region where a window region is to be formed in which the ZnO layer 201 on the active layer 104 is formed, and the p-GaAs contact layer 109 is removed from most of the gain region. As described in the specification or the drawings of Japanese Unexamined Patent Publication No. 2006-319120, in portions other than the window, the subsequent manufacturing process is performed with the surface of the p-type GaInP clad layer 107 being exposed.
However, it is known in this technical field that a defect is more likely to occur in the vicinity of the surface of a layer made of GaInP or AlGaInP than a layer made of GaAs due to, for example, a heat treatment or a plasma process. Therefore, a crystal defect is likely to occur in the exposed p-type GaInP clad layer 107 due to a thermal history of a manufacturing process. The defect is spread into a crystal by the heat treatment, and interdiffuses elements forming the crystal. Therefore, in the active layer 104 in the gain region, alloying occurs due to the interdiffusion of the crystal, which may result in a variation in band gap.
Even though the diffusion of Zn in the lateral direction is prevented, it is difficult to prevent the alloying of the active layer in the gain region where the p-GaAs contact layer 109 is not provided with high reproducibility.
Although the band gap of the active layer is not described in detail in Japanese Unexamined Patent Publication No. 2006-319120, the band gap of the active layer is likely to be widened due to alloying in the gain region where the ZnO layer 201 is not formed.
Japanese Unexamined Patent Publication No. 2007-318077 discloses an improved semiconductor laser that prevents the generation of a crystal defect due to process damage.
FIGS. 17A to 17D show a process of manufacturing the semiconductor laser. First, an n-type AlGaInP clad layer 3, an active layer 4, a first p-type AlGaInP clad layer 5, a p-type etching stop layer 6, a second p-type AlGaInP clad layer 7, a p-type barrier reduction layer 8, and a p-type GaAs cap layer 9 are sequentially formed on an n-type GaAs substrate 2, and the p-type GaAs cap layer 9 near the end surface of a resonator is removed to form an opening. Then, a ZnO layer 11 is formed in the opening, and a heat treatment is performed to diffuse Zn included in the ZnO layer 11 into the active layer 4, thereby forming a window region M. Then, a strip-shaped insulating film mask pattern 16 is formed in a resonator direction so as to cover the window region M. Then, only the p-type GaAs cap layer 9 is removed by a selective etching solution to form a ridge portion 17. According to Japanese Unexamined Patent Publication No. 2007-318077, it is possible to improve a manufacturing yield.
In a method of manufacturing the semiconductor laser disclosed in Japanese Unexamined Patent Publication No. 2007-318077, as shown in FIG. 17D, a heat treatment is performed while the ZnO layer 11 and the p-type GaAs cap layer 9 are provided on the same layer so as to come into contact with each other, thereby diffusing Zn in the ZnO layer 11 into the active layer 4. In this case, Zn is diffused in the resonator direction through the p-type GaAs cap layer 9. As a result, a non-gain region of the active layer is widened, which causes an increase in threshold current value.
When the volume of the non-gain region is large in the active layer, a sufficient gain is not obtained, which results in an increase in threshold current value. In particular, this phenomenon is remarkable in the laser that has a relatively small resonator length and is for playing back an optical disk. In addition, in devices using a fine balance between the gain and the loss, such as self-oscillation lasers, it is very difficult to generate self-oscillation.
As described above, the related art disclosed in Japanese Unexamined Patent Publication Nos. 2006-319120 and 2007-318077 has the following problems.
First, since impurities are diffused in the resonator direction of the active layer, the non-gain region is widened, which causes an increase in threshold current value.
Second, since a crystal defect occurs due to process damage, it is difficult to control the alloying of the active layer with high reproducibility.